Make sure your OpenLDAP build, etc, has debugging symbols.
I've never done this, so just to be sure, to do this I need to pass
CFLAGS="-g -O0" when running configure, then make install STRIP="" when
it's time for that step, correct?

Correct

Then, I simply "gdb /path/to/slapd [options] /path/to/slapd.core"?

Correct

Lastly, do I need to change the slapd log level at all, or will all of
the helpful information be in the core file?